1. Field of the Invention
The present invention relates to a method of interfacing a high-speed signal, and more particularly to a method of interfacing a high-speed signal, capable of reducing crosstalk between adjacent channels.
2. Description of the Related Art
An electric signaling system has a transmitter, channels (or interconnects) and a receiver. Generally, a channel is composed of copper, and a signaling interface receives and transmits digital signals. The signaling system may be, for example, applied to computer-to-peripheral connections, local area networks, memory buses, multiprocessor interconnection networks, etc. As operating speed level and circuit integration level of semiconductor chips increase, a required bandwidth for off-chip data also increases so as to permit transmission and reception of data at high frequency. Therefore, there are design issues of the signaling interface such as high-speed operation, immunity to noise, clock generation/timing recovery, inter-symbol interference, and crosstalk, etc.
Crosstalk is a phenomenon in which a signal transmitted through one of multiple channels causes undesired noise to a neighboring channel.
FIGS. 1A through 1E are waveform graphs illustrating crosstalk that occurs between two adjacent channels.
FIG. 1A shows a pulse signal V1 that is applied to a first channel. FIG. 1B shows a noise signal V2S that occurs at a near end of a second channel due to the pulse signal V1 of the first channel, when there exists only a capacitive coupling between the first and second channels. FIG. 1C shows a noise signal V2E that occurs at a, far end of the second channel due to the pulse signal V1 of the first channel, when there exists only a capacitive coupling between the first and second channels.
In addition, FIG. 1D shows a noise signal V2S that occurs at a near end of the second channel due to the pulse signal V1 of the first channel, when there exists only an inductive coupling between the first and second channels. FIG. 1E shows a noise signal V2E that occurs at a far end of the second channel due to the pulse signal V1 of the first channel, when there exists only an inductive coupling between the first and second channels. As shown in FIGS. 1A through 1E, when a signal of a channel transitions, noise occurs at a neighboring channel.
FIG. 2 is a schematic diagram illustrating an exemplary four-level pulse amplitude modulation (PAM) system where two-bit binary values are assigned to voltage levels using a gray code, and FIG. 3 is a schematic diagram illustrating an exemplary eight-level PAM system where three binary values are assigned to voltage levels using a gray code.
When pulse amplitude modulation (PAM) is used to transfer data, transfer speed may increase. For example, six channels are required to transfer six-bit data without the PAM, while a 4-PAM transfers the six-bit data using three channels and an 8-PAM transfers the six-bit data using two channels. This is because two-bit data may be transferred via one channel using the 4-PAM and three-bit data may be transferred via one channel using the 8-PAM.
Referring to FIGS. 2 and 3, when a voltage level difference between adjacent data bits is represented by Δ, a maximum voltage level difference in data transition results in 3Δ in the 4-PAM, and 7Δ in the 8-PAM. However, when the voltage level difference in data transition is 7Δ, crosstalk between adjacent channels becomes serious.
Therefore, although the transfer rate may be increased in the 8-PAM, it is disadvantageous in that serious crosstalk may occur in the 8-PAM.
A method of canceling the crosstalk is adding a compensation signal to an interfered interconnect. However, the method is sensitive to process, temperature and interconnect parameter variation, etc.
Another method of canceling the crosstalk is providing a pair of interfering interconnects that are nearest neighbor interconnects of the interfered interconnect. A signal of one of the pair of interfering interconnects is symmetric to that of the other of the pair of interfering interconnects. The method is robust to the variations in process, temperature and interconnect parameters, etc. However, the method needs to employ dummy interconnects, and the transfer rate may be greatly decreased.